Transient spectroscopy is used to study the kinetics of conformational changes in macromolecules subsequent to excitation with a pulsed laser. Changes in both the tertiary and quaternary structure of hemoglobin have been observed following the photodissociation of carbon monoxide from the hemes. Steady state photodissociation of carbon monoxide from hemoglobin S is used to study the nucleation-controlled kinetics of the assembly of deoxy hemoglobin S into polymers. This technique has recently been modified by adding an additional laser beam which can be used to prepare a partially saturated sample by photolysis of CO hemoglobin S or to probe the kinetics of the sample by modulation techniques and a pump and mixer system which permits cells to be equilibrated with solutions of known oxygen tension. A second, high intensity beam can then be used to remove all of the ligands and measure the polymerization kinetics of the resulting deoxy hemoglobin S sample. We have begun to use this system to explore the binding of CO to hemoglobin in the photo stationary state produced at different levels of photolysis and to measure ligand binding curves to single sickle cells.